this adds basic support for loading mods from spacedock.info
this currently does not handle dependencies or dependency resolution
there are issues with the new package loading design that need to be resolved

Add a loading spinner to indicate when packages are refreshing
Add a Resize thumb between package list and current package view

Unity versions lower than 2020.1 do not support using Assembly Definition references via GUIDs and must use string matches.

The GameObject being destroyed after an image was loaded was leading to other loads being cancelled.
There is no need to manually destroy this game object because it is created as HideAndDontSave and will therefor automatically be destroyed by unity when the domain unloads

Images will be loaded into memory and never unloaded when downloaded using the MarkdownElement.
When the Image element is created for MarkdownElement, we will attach a DetachFromPanelEvent to it so that it will have Unity destroy the Texture object.

Utilize MarkdownElements for the Package Header, Body, and Footer sections so that their content is more customizable per PackageSource.
Additional work needs to be done here to improve the workflow and modularity of this facet.

This should resolve some issues with unnecessary prompts, and also enable each to enabled independently of eachother

Images were always loading from their sources.  Images will now cache instead, reducing latency and network usage

The ImageElement code for VisualElementFactory became too substantial for a clean class, so it has been split off into its own factory class

Add display showing image cache size, file count, and a button to clean the cache

AddressablePreviewStage provides the ability to load an addressable asset onto a custom Stage for viewing Addressable Prefabs.
This provides access to the prefabs transform hierarchy and components.

… matching switches

…19.4-

This likely is incomplete, but this will allow the games addressable catalog to load and to load assets from the games library while the Addressables Package is installed.

This may interfere with Addressable building functionality, this needs to be investigated heavily

…xecution of the last importer on every domain reload

Update AddressablePreviewStage so that it is only compiled when TK_ADDRESSABLE is a defined symbol

We just don't care, if the delete fails we try to overwrite, if that fails, we can't do anything about it anyways